This invention relates to a thermoset bonding agent for non-distortion joining of self-supporting thermoset component parts.
Rigid component parts of a thermoset plastic resin may be joined by different means including mechanical connections, and/or chemical bonds using various adhesive and bonding agents. A fiberglass material of a known thermoset plastic resin mixed with fiberglass forms a high strength sheet element. Such material and elements are useful as a replacement for metal components because complex parts can be conveniently formed and shaped with a highly finished surface. Fiberglass reinforced components may be formed which are lighter than comparable metal parts, and are not of course subject to the usual corrosion which is so destructive of metal parts in motor vehicles or the like. The complex parts also reduce the number of parts required and may result in significantly less total tooling costs.
The parts must of course be joined with a long-life and high strength connection. In the fabrication of metal parts, sophisticated and excellent joining methods such as the various forms of welding have been highly developed and provide rapid, low cost production of metal parts. Plastic components have generally presented more difficult, or expensive joining problems.
Various products are now made of a special glass reinforced epoxy or other thermoset resin, which is identified as an SMC product, from the sheet-molding-compound material of which it is formed. In such products, one method of bonding is a thin epoxy adhesive of the two part type. Conventionally, the epoxy adhesive includes an epoxy resin and a curing agent which cures the adhesive and which is known to be temperature dependent. To accelerate the curing reaction, the assembly may be coupled to a heating unit under pressure. The heat is typically applied, in commercial practice, for a period such as six minutes to heat the elements and transfer the heat to the adhesive and thereby generate a partially bonded assembly.
In particular, the adhesive bonding is widely used in automotive and marine products for joining of the fiberglass and similar parts. In the prior art bonding methods one or more beads of adhesive are applied to a first sheet molded compound member and the second member applied thereon. The members are clamped between a heated platen and a pressure plate means, which serves to flatten the adhesive so as to cover the intended bond or joint surfaces. The pressure pad may be applied after the curing has begun and is normally held until a "gel" condition has been established.
The pressure pad is applied so as to spread the adhesive to establish a selected thickness of adhesive. The heating system heats the component parts which transmitts the heat to the adhesive to increase the curing rate of the adhesive. Various adhesives such as an epoxy or a polyurethane are used in such joining processes in both the automotive and the marine industries. Generally, the adhesive is a two component mixture, one of which activates a curing reaction in the mixture to cure to a solid adhesive mass firmly bonded to the adjoining elements. Although the adhesive will generally cure at room temperature, the time required is generally on the order of two or more hours which is generally prohibitively long for commercial implementation in mass production, such as in marine and vehicle production systems. Accelerated curing is therefore uniformly created by heating of the elements to raise the temperature of the elements and the adhesive, while holding of the parts under pressure to establish and hold the adhesive layer equal typically to 0.03 inches.
The assembly is heated for a sufficient period to increase the temperature until a "gel" condition is reached, which the inventor has found is typically a state in which the adhesive joint has a strength of 100 PSI (per square inch). The "gel" condition is more generally defined as that partially cured state which is sufficient to permit subsequent handling of and processing of the assembly in the production system. In one application, a "cure" time of at least six minutes is required to create a "gel" condition, or bond strength of 100 PSI. The hot elements of course contribute to further curing during such subsequent processing.
Such prior art bonding methods generally require the relatively long processing time of six minutes or more because of the heat transfer characteristic of the element. Further, the heat in the elements tends to spread from the bond area into the adjacent portions which involves heating of substantially greater mass than necessary for the adhesive curing. Further, heating of the adjacent portions is known to increase the temperature to a distortion related level at which the physical characteristic of the element may be adversely affected and physical distortion may occur directly or during subsequent handling.
The present inventor has been involved in an improved thermoset curing system, such as disclosed in
U.S. Pat. Nos., 3,461,014 which issued Aug. 12, 1969 and 3,730,806 which issued May 1, 1973 wherein the adhesive is mixed with a heat generating particulate for a more rapid and improved heating characteristic. The disclosed embodiment therein is particularly related to a non-rigid element such as the binding of books. Although such system may be used to advantage, the inventor has found several factors are critical to bond self-supporting and generally rigid elements of a sheet-molded-compound material and particularly to provide practical implementation of an accelerated curing process in commercial-type production of such rigid plastic elements using a thermoset adhesive.